Seeing as its a anatomy test, I'd change his normal hand to hold the cannon, not his upper arm. Whether for support (by holding the underside) or to counteract the recoil (works both top and bottom), it makes more sense to hold the weapon itself.

I see your point, originally I wasn't leaning towards the whole "hand cannon" concept and had him hold his upper arm to practice a sense of distance and perspective of limb positioning... but when I got around to it, I was like "meh" on the need to re-position his arm...

The thought came from watching Sword Art Online to where I wondered if a player was capable of dropping a weapon based Item and being able to auto-equip another in a manner to which he/she could slash/throw one weapon while having multiple weapons suspended in the air from tossing it up/ or just simply lying about and grabbing/auto-equiping that weapon to continue attacking or to change fighting styles at a very high pace... without having to access his game menu in combat.